The present invention relates to electrophoretic cells that form an electrophoretic display. In particular the invention relates to a stacked cell configuration for use in a color electrophoretic display operating in a light-reflective mode.
An electrophoretic cell is a cell comprised of pigment particles suspended in a fluid and uses electrophoresis to switch between the following two states:
Distributed State: Particles are positioned to cover the horizontal area of the cell. This can be accomplished, for example, by dispersing the particles throughout the cell, by forcing the particles to form a layer on the horizontal surfaces of the cell, or by some combination of both.
Collected State: Particles are positioned to minimize their coverage of the horizontal area of the cell, thus allowing light to be transmitted through the cell. This can be accomplished, for example, by compacting the particles in a horizontal area that is much smaller than the horizontal area of the cell, by forcing the particles to form a layer on the vertical surfaces of the cell, or by some combination of both.
The electrophoretic cell can serve as a light valve since the distributed and collected states can be made to have different light absorbing and/or light scattering characteristics. As a result, an electrophoretic cell can be placed in the light path between a light source and a viewer and can be used to regulate the appearance of a picture element or xe2x80x9cpixelxe2x80x9d in a display. The basic operation of reflective electrophoretic cells along with the examples of various electrode arrangements are described in IBM""s U.S. Pat. Nos. 5,745,094 and 5,875,552.
Reflective color displays are known that use liquid crystals in conjunction with a fixed polarizer element to control the intensity of light reflected form each pixel. Since polarizers absorb the fraction of light whose polarization is not aligned with their active axis, and since this absorption varies with the angle of incidence, displays based on their use suffer from both limited reflectivity and viewing angle.
Other reflective color displays are known that use a solution of a dichroic dye in single or multiple layers of either a nematic or cholesteric liquid crystal material. Using a single nematic layers requires the use of a fixed polarizer element and therefore suffers from the aforementioned limitations. Using one or more cholesteric layers, or more than one nematic layer, eliminates the need for a fixed polarizer element and increases the achievable reflectivity. This approach still relies on the selective absorption of polarized light and, as a result, the contrast changes with viewing angle.
Other reflective color displays are known that use scattering liquid crystal materials, such as polymer-dispersed liquid crystal materials or scattering-mode polymer stabilized cholesteric texture materials, to control the intensity of light reflected from each pixel by switching between a turbid state and a uniform state. Since these materials only weakly scatter light in their turbid state, reflective displays based on them have a low diffuse reflectance and therefore also suffer from low brightness.
Other reflective color displays are known that use reflecting liquid crystal materials, such as reflective-mode polymer-stabilized cholesteric texture materials or holographic-polymer-dispersed liquid crystals, to control both the intensity and color or reflected light reflected from each pixel via diffraction effects. Since these depend on diffraction effects, it is difficult to simultaneously achieve large viewing angle, high reflectance, and angle independent color.
Liquid crystal displays commonly use a side-by-side arrangement of single color subpixels within each pixel to generate color via spatial color synthesis. The reflection efficiency of such an arrangement is limited by the fact that each subpixel only occupies a fraction of the total pixel area. By arranging the subpixels in a vertical stack, each subpixel can occupy the same lateral area as the pixel itself, and the reflection efficiency can be significantly increased. U.S. Pat. No. 5,625,474 assigned to the Sharp Corporation and IBM""s U.S. Pat. No. 5,801,796 describe embodiments of stacked cell arrangements suitable for liquid crystal materials. Since these embodiments rely on liquid crystal materials, however, they remain hindered by the aforementioned limitations of such materials. Electrophoretic displays do not suffer from these limitations and can offer improved reflection characteristics combined with extremely low power requirements.
Reflective color electrophoretic displays have been proposed in the prior art. Japanese Patent JP 1267525 assigned to Toyota Motor Corporation describes an electrophoretic display having colored (blue and yellow) particles with different zeta potentials in a solution of red dye to give a multicolored (yellow, green and red) display. When a certain voltage is applied to the pixels, the yellow particles are pulled to the front transparent electrode and the viewer sees yellow. At a higher voltage, the blue particles are also pulled to the front electrode and the viewer sees green. When the particles are pulled off the transparent electrode, the colors of the particles are hidden by the dye solution and the viewer sees red.
U.S. Pat. No. 3,612,758 assigned to Xerox Corporation describes a reflective electrophoretic display having pigment particles of a single color in a contrasting dye solution. In this scheme, under the influence of an electric field, the particles migrate to a front transparent electrode and the viewer sees the color of the particles. When the field is reversed, the particles migrate away from the front transparent electrode, are hidden in the dye solution, and the viewer sees the color of the dye solution.
In the two electrophoretic display patents above, color contrast and reflectance depend on the presence or absence or particles at the front window. Since the dye solution can not be completely removed from the space between the particles when they are at the front window, displays based on this approach do not produce high contrasted images and generally have a low reflectance.
WO 94/28202 assigned to Copytele Inc. describes a dispersion for a reflective electrophoretic display comprised of two differently colored particles that are oppositely charged. The polarity of the voltage applied to the cell determines the polarity of the particle attracted to the front transparent electrode, and hence determines the color seen by the viewer. Since the viewer sees either one of two colors, this approach produces monochrome images and therefore has a limited color gamut.
U.S. Pat. No. 5,276,438 assigned to Copytele Inc. describes a reflective electrophoretic display in which a mesh screen, disposed behind the front window and covering the viewing area of the display, is used either with or without a dyed suspension fluid to hide particles of a single color from the viewer. When the particles are positioned in front of the mesh, the viewer sees the color of the particles. When the particles are positioned behind the mesh, the viewer sees a mixture of the mesh and particle colors. As a result, the contrast produced by this approach is limited by the open area of the mesh. In addition, this approach produces monochrome images and therefore has a limited color gamut.
U.S. Pat. No. 3,668,106 assigned to Matsushita Electric describes reflective electrophoretic displays using white or colored electrophoretic particles in colored or transparent suspension media in a side-by-side arrangement of colored subpixels to generate color via spatial color synthesis. IBM""s U.S. Pat. No. 6,225,971 and pending application 09/154,284 also describe reflective electrophoretic displays that rely on spatial color synthesis, but which have improved reflectivity and color gamut. As stated above, however, the reflection efficiency of color generation via spatial color synthesis is limited by the fact that each subpixel only occupies a fraction of the total pixel area.
There is a continuing need in the art for a low-power reflective color display with high reflectance, high image contrast, and large color gamut. It would be desirable, therefore, to incorporate the advantages offered by electrophoresis in a scheme that can utilize vertically stacked subpixels to maximize reflection efficiency. Electrophoretic displays that rely on hiding particles in a dye or behind a mesh are not suitable for stacking, since their reflectivity and contrast originate from the need to prevent light from passing through both the particles and the hiding medium. Furthermore, stacked cell structures suitable for liquid crystal materials are not appropriate for stackable electrophoretic schemes. In particular, the lateral parallel-plate electrode geometries used in both stacked and non-stacked liquid crystal displays are not capable of switching an electrophoretic suspension between its distributed and collected states. In addition, since electrophoretic suspensions can be influenced by weak electric fields, such geometries do not provide sufficient isolation of any given subpixel from the stray electric fields that originate from its neighbors.
The present invention is a reflective color electrophoretic display. The display is intended to be viewed while illuminated from the front window by ambient light and to operate without the need of a backlight. The display is comprised of many picture elements or pixels located in lateral adjacency in a plane. Each pixel is comprised of two or more subpixels, or cells, which are vertically stacked, one directly above the other on a reflective panel located at the rear or bottom of the stacks. The cells contain a light-transmissive fluid and charged pigment particles that can absorb a portion of the visible spectrum, with each cell in a stack containing particles having a color different from the colors of the particles in the other cells in the stack. The color of a pixel is determined by the portion of the visible spectrum that survives the cumulative effect of traversing each cell in the stack and reflecting off the reflective panel. Each cell is comprised of light-transmissive front and rear windows, at least one non-obstructing counter electrode, and at least one non-obstructing collecting electrode. A plurality of vertical side walls extend from the rear panel and support the windows in a spaced apart relationship. The side walls are vertically aligned with one another and thus divide the display into a plurality of vertical stacks of cells, each stack forming a pixel. The electrodes are controlled by solid state switches or driving elements, such as a thin film transistor or a metal-insulator-metal device, formed on the inside surface of the rear panel, with electrical connection being made vertically through holes in the windows that separate the cells in the stacks.
The amount and color of the light reflected by each cell is controlled by the position and the color of the pigment particles within the cell. The position, in turn, is directed by the application of appropriate voltages to the collecting and counter electrodes. When the pigment particles are positioned in the path of the light that enters the cell, the particles absorb a selected portion of this light and the remaining light is transmitted through the cell. When the pigment particles are substantially removed from the path of the light entering the cell, the light can pass through the cell, reflect from the reflective panel, pass through the cell again, and emerge without significant visible change. The light seen by the viewer, therefore, depends on the distribution of particles in each of the cells in the vertical stacks. Since each of the cells or subpixels in the stack occupy the same lateral area as the pixel itself, the reflection efficiency can be significantly higher than that of embodiments that rely on a side-by-side arrangement of subpixels to generate color.
A more thorough disclosure of the present invention is presented in the detailed description that follows and from the accompanying figures.